The invention relates to a retention system used to prevent axial and/or radial movement of a shaped object in a correspondingly shaped slot and, more particularly, to a retention system and method for retaining a turbine blade dovetail in a retention ring dovetail slot.
In a conventional turbine compressor component, stator blades are held in a retaining ring by means of a dovetail connection (i.e., a dovetail on the blade is received in a complimentary slot in the retaining ring), and the retaining ring, in turn, is secured within a circumferential slot in the compressor casing.
The fit between the blade and the dovetail slot in the ring is loose to allow for assembly and tolerances. Therefore, if the blades are not properly retained, the loose fit may allow the hardware to move in the slot, leading to excessive wear. The excessive wear would eventually fail the part, requiring the unit to be shut down until a repair can be made.
Additionally, radial movement of the blade in the dovetail slot allows for a variation of the tip radius during the machining process. Reduced variation in radial clearance could potentially increase performance and avoid excessive tip rubs.
Typically, each stator blade is retained in the ring to limit motion along the ring dovetail slot by one or more stakes. This is a process where material at the edge of the ring slot is plastically deformed and displaced into a void created by a local chamfer of the stator dovetail. This is a manual and highly variable process which can in some cases provide inadequate retention of the stator blade in the ring slot. Vibratory forces acting on the stator can produce wear on the stake leading to eventual failure of the retention feature. Once the stake is worn, the blade can then slide freely in the ring slot. At very high amplitudes, this motion can lead to wearing of the ring dovetail and eventual failure of the ring. This could then lead to blade liberation and subsequent collateral damage to the gas turbine. This problem has been addressed in recent designs of the assignee via a set screw solution, disclosed in commonly-owned, pending application Ser. No. 11/282,603, filed Nov. 21, 2005. There have also been many documented instances of stators being installed incorrectly either by inserting the airfoil in the ring backwards or inserting the stator or ring in the wrong axial position (stage). Some of these mis-assemblies have been identified as causes of subsequent failure of machine equipment.
There remains a need, however, for a reliable, mistake-proof retention technique for securing airfoil stator blading in turbo-machinery.